Color CRTs employ an electron gun arrangement that generates three electron beams, one for each of the primary colors of red, green and blue. A common approach is to arrange the three electron beams in an inline array and to simultaneously scan the three electron beams in a raster-like manner from the top to the bottom of the CRT's display screen. The shadow mask includes hundreds of thousands of small electron beam passing apertures and serves as a color selection electrode permitting each electron beam to be incident only upon its associated color producing phosphor elements on the inner surface of the CRT's display screen. The shadow mask and display screen are typically provided with a spherical curvature in an axially symmetric fashion and are arranged in closely spaced relation in the CRT,
The large number of electron beam passing apertures in the shadow mask are arranged in a closely spaced manner. The spacing between adjacent apertures is defined in terms of a vertical pitch, i.e., the vertical center-to-center spacing between adjacent apertures in the same vertical column, and horizontal pitch, i.e., the horizontal center-to-center spacing between adjacent apertures in the same horizontal row. FIG. 1 is a simplified plan view of one prior art approach showing a shadow mask 11 incorporating electron beam passing apertures 13 with a constant P.sub.v In the prior art shadow mask 11 of FIG. 1, the horizontal pitch P.sub.h is constant for all rows of apertures from the mask's Y-axis out to the lateral edges of the mask. The vertical pitch P.sub.v is also constant for all columns of apertures from the Y-axis out to the lateral edges of the mask.
Because of the curved geometry of the shadow mask and the display screen and because the electron beam is directed from a fixed point through the mask's apertures onto the phosphor elements on the display screen, the mask apertures and screen phosphor elements are not in alignment with respect to the electron beam over the entire surface of the display screen. In particular, misalignment between the beam passing apertures and the phosphor elements is most severe in the four corners of the display screen.
In the past, the primary emphasis in aperture spacing has been in terms of the horizontal pitch P.sub.v The reason for the emphasis on P.sub.v is because this parameter primarily determines the curvature of the mask, as well as its strength and stiffness. Until recently, shadow mask designers have not emphasized the vertical pitch P.sub.v of the mask apertures in shadow mask design.
More recent shadow mask designs employ a vari-bow design as shown in FIG. 2 which is a plan view of a shadow mask 15 incorporating a plurality of electron beam passing apertures 17 arranged in a vari-bow configuration. As in the prior art shadow mask discussed above, the horizontal pitch P.sub.h is constant over the entire surface of the shadow mask. The vari-bow design defines the vertical pitch P.sub.v in terms of the following expression: EQU P.sub.v (x)=P.sub.vo .times.F(X) (1)
where
P.sub.vo =vertical pitch at the Y-axis where X=0, and EQU F(X)=1+aX.sup.2, (2) PA2 with a=constant less than 0.
The vari-bow design facilitates CRT manufacture and assembly by increasing the color purity adjustment margin in the corners of the display screen. However, the vertical separation between adjacent trios of red, green and blue phosphor elements on the display screen's inner surface is reduced, resulting in a degradation in video image resolution in the corners of the screen. This is because P.sub.v decreases monotonically along the X-axis in accordance with Equation (1) and is minimum in the four corners of the display screen. This reduced video image resolution in the display screen corners of a color CRT employing a vari-bow shadow mask aperture arrangement is particularly severe in high resolution displays which are gaining increasingly greater commercial acceptance.
The present invention addresses the aforementioned limitations of the prior art by providing a shadow mask aperture arrangement for use in a color CRT which affords increased video image color purity adjustment margin without sacrificing video image resolution particularly in the corners of the CRT's display screen.